The invention relates to a method and an internal combustion engine suitable for carrying out this method, in which an intake airflow for the internal combustion engine and a secondary airflow for injection into the exhaust system of the internal combustion engine are simultaneously adjusted with respect to the required mass flow.
Apparatus of the kind referred to above are known, e.g., from WO 97/38 212. According to FIG. 8 of this document a secondary air injection system for an internal combustion engine is presented, which consists of a turbine unit 114 and a compressor 113. The turbine is driven through a bypass duct which is arranged parallel to the throttle valve 115 in the air intake tract. In the bypass duct to the turbine there is furthermore disposed at least one throttling member 120. The simultaneous adjustment of the air intake stream and secondary air stream is performed by the interaction of throttle valve 115 and throttling member 120. Thus on the one hand the power of the turbine 114, and thus also the secondary air delivered by the compressor 113, can be adjusted, and on the other hand the delivered intake air stream can be adjusted as an addition of the air streams by the throttling member 120 and the throttle valve 115.
For the optimum adjustment of the two air streams the complex actions of the internal combustion engine must be known. On this basis the current air requirement in the intake tract of the internal combustion engine and in the exhaust system can be determined. The air requirement of the internal combustion engine is dependent, for example, on the load state, but also on the desired kind of operation, e.g., the combustion of the fuel with excess oxygen or oxygen deficiency. Secondary air is introduced into the exhaust system in the cold-start phase of the motor, for example. This is intended to oxidize incompletely burned exhaust components and to additionally heat by this exothermic reaction the catalyst that follows in the exhaust system. Thus, during the cold-start phase the pollutant discharge is reduced, and the cold-start phase is shortened, since the catalyst goes into action sooner due to the heating.
In the introduction of secondary air a certain air ratio must be established so that the emissions reduction will operate. In case of too much secondary air the exhaust gas is cooled too strongly without the oxidation of the additional exhaust components. In the case of too little secondary air not enough oxygen is available to oxidize the exhaust components.
One object of the invention is to provide a method that will enable a satisfactory adjustment of the secondary airflow and the intake airflow for the internal combustion engine by means of simple principles. A further object of the invention is to provide suitable devices for this method.
The method of the invention is appropriate in a known manner for the simultaneous adjustment of the intake air stream of the internal combustion engine and of the secondary air stream in the exhaust system of the internal combustion engine. The adjustment of the air streams is performed through a variation of the mass flow. In the air intake tract of the internal combustion engine a throttle valve is contained, the latter being able to affect only the mass flow of the intake air in the intake tract. When the intake air stream is adjusted, allowance must be made for the ambient air flow which is branched off to drive the turbine ahead of the throttle valve and to be returned to the intake tract behind the throttle valve. The turbine drives a compressor which produces the secondary air stream. The latter is fed to the exhaust system, and its injection ahead of the catalyst, for example, is desirable in order to achieve the previously described effects for the exhaust gas in the cold starting phase of the engine.
The invention is characterized in that at least when the internal combustion engine is idling the throttle valve assumes a position established for this state. This is done in view of the fact that, when the internal combustion engine is idling the need for air is known. By putting the throttle valve in a set position and putting the known air intake performance of the internal combustion engine at idle, it is thus possible simultaneously to achieve a defined turbine power. The throttle valve must be adjusted such that the air intake stream which sweeps past the throttle valve when added to the ambient air stream results in just the amount of air necessary for the idling of the internal combustion engine. In this way it can be brought about that, in this state of operation, the internal combustion engine runs with the required rotatory speed. Fluctuations in the amount of intake air delivered would result in an irregular speed of the internal combustion engine.
The valve position desirable for the idling speed, in relation to the air stream required for ordinary driving, must be determined for the internal combustion engine. For this purpose a throttling means can additionally be used, which is arranged in the bypass line and by means of which the air stream at the turbine can be influenced. The known conditions when the internal combustion engine is idling render unnecessary therefore a regulation or control of the rate of flow of the intake air. In this state of operation, slight departures from the intake air rate of flow that is actually desired are especially critical, since the internal combustion engine reacts to them immediately with a change in rpm. In other operating conditions, departures from the desired amounts of air intake and secondary air flows are of less importance. For these can be established in the bypass duct without providing any additional adjusting means such as the throttle valve and the throttling device as will be explained more precisely below. This has a positive effect on the economy of the proposed method, since additional regulating means become superfluous. The throttle valve setting can be operated, for example, through the motor control that is present anyway.
According to an additional embodiment of the invention, the established position of the throttle valve in the idling state is precisely when it is closed. In this case the air intake for the internal combustion engine is supplied exclusively through the bypass line. In that case the turbine can be used as the means for controlling the amount of intake air during idling. This becomes possible because the air throughput by the turbine is dependent upon the power consumed by the compressor, although in this case care must be taken that the compressor power is simultaneously affected by the mass flow of the secondary air.
According to an improvement of the invention, the throttle valve is to remain in the closed position as long as the required bypass air flow of the internal combustion engine is below the maximum possible at the turbine. Because this is the condition required that the air demand of the internal combustion engine can be covered exclusively through the bypass duct. In these circumstances of operation the maximum possible turbine power is always available. It can thus be converted to the greatest possible secondary air stream, which increases the possibilities for the variation of the amount of secondary air fed into the exhaust system.
If the turbine output is too great in the case of the complete closure of the throttle valve, to achieve a satisfactory setting of the secondary air flow, a throttle valve can be provided in the secondary air duct, according to a special embodiment of the invention. This permits reducing the secondary air flow whenever the mass of air being delivered is too great for the momentary state of operation of the internal combustion engine. This effect can advantageously also achieved by an additional duct which bypasses the compressor. This is made so as to be throttled and it can also be completely shut off. In the case of an opening, an air stream is produced in the additional duct which is opposed to the flow delivered by the compressor and while the compressor power remains the same it reduces the secondary air stream effectively delivered to the duct leading to the exhaust system.
The additional duct is especially able to reduce the secondary air stream independently of the turbine power. In this case it must be noted that the compressor output as well as the turbine output are coupled directly to one another. If the secondary air stream is reduced by a throttle in the secondary air duct, this will also result in a reduction of the throughput of air to the turbine. In case the internal combustion engine requires a greater amount of air, this must be offset by an adjustment of the throttle valve in the air intake tract. Thus it also appears that a throttle valve present in the secondary duct leads at the same time indirectly to a throttling of the admixed air to the turbine. In this case throttling of the secondary air stream can be dispensed with.
An internal combustion engine on which the method of the invention is to be carried out must at least have the following known components. It must be provided with an exhaust system with a catalyst, since the catalyst makes the secondary air supply necessary. A throttle valve must be present in the air intake tract of the internal combustion engine while a turbine is provided through a bypass line parallel to the throttle valve, which likewise can carry the intake air. The turbine is mechanically coupled to a compressor, so that the turbine power can be utilized as in a pumping power of the compressor for the introduction of the secondary air into the exhaust system [translation doubtful]. Furthermore, means must be provided for the performance of the already described process.
These means can consist, for example, of a throttle device which is arranged in the bypass duct and again is coupled mechanically with the throttle valve in the intake tract. By this mechanical coupling a fixed law is produced as to how the degrees of opening of the two throttle devices are related to one another. Especially, a linear relationship is achievable which allows for the circumstance that an increased need of air by the internal combustion engine requires a augmented supply of secondary air through an increased mass flow of the exhaust gas. The secondary air mass flow can thus be kept substantially constant. The throttle device for the bypass duct can consist, for example, of a sliding door which can be shifted back and forth in the wall of the air intake tract and in this manner control an opening connecting the bypass duct and the air intake tract. The sliding door can be continuously varied so that partial closing of the bypass line becomes possible. It is also conceivable that the sliding door have several passages through it which slide step-wise past the through opening, Also a plurality of connecting ports can be arranged, in which case the branching of the bypass line to the ports is necessary. The sliding door can be driven, for example, by a toothed rack mounted thereon. The mechanical coupling of the throttle valve is achieved, for example, by a toothed wheel which is mounted on the throttle valve shaft and meshes with the toothed rack.
With the embodiment described it is possible to establish especially a linear relationship between the throttle valve in the air intake tract and the throttle device in the bypass line. Of course, other drives are conceivable, which among other things can bring about also a degressive or progressive relationship between the throttle valve position and the throttle device position. Thus special air intake characteristics can be achieved in internal combustion engines, which in the individual case can be adapted to the given circumstances of the motor.
Another variant of the throttle means in the bypass duct is obtained by using a control valve for the throttling. This control valve communicates through a connecting line with the secondary air duct, so that the information on the compressor pressure applied in the secondary air duct can be fed into the valve through a control connection in the valve. The secondary air pressure can thus be used directly as a control variable in order, for example, to influence the turbine power by a corresponding adjustment of the control valve so that a constant secondary air pressure is formed. As soon as the secondary air pressure increases above the desired level, the control valve in the bypass duct is throttled, so that the turbine power decreases and with it the compressor power, which leads to a lower secondary air pressure. The control valve can also be designed such that any pressure pulsation in the secondary air duct or bypass duct is absorbed by damping factors which are associated with the design of the control valve.
In case of the use of the control valve described, its position depends only on the required secondary air pressure. In order at the same time to assure an optimum supply of intake air to the internal combustion engine the intake air stream must therefore be regulated by the position of the throttle valve. Another possibility is that control valves also control through the pressure present in the air intake tract, so that the latter, as a controlling factor, influences the action of the control valve in order to find a position constituting a compromise regarding the mass flow in the secondary air duct and in the intake duct of the internal combustion engine. In this case too, the marginal conditions of the internal combustion engine that is idling can be assumed to be known, so that the throttle valve assumes a position appropriate for this state of operation.
According to another embodiment of the invention, the method described in the beginning can be achieved by means of a control unit which has an input for a sensor in the secondary air duct, an input for a position sensor at the throttle valve and an output for a control signal for a control valve for controlling the secondary air volume. The sensor in the secondary air duct can be designed to detect the pressure present in the secondary air duct, or also the throughput of secondary air. The corresponding secondary air signal can be evaluated by the control and gives an indirect or direct information on the amount of secondary air delivered to the air intake tract. In modern internal combustion engines an indication of the position of the throttle valve is provided in order to achieve optimum motor control. This knowledge of its position can be used simultaneously by the control unit for the secondary air system in order to obtain indirect information on the amount of air that can be drawn from the air intake system. The position indication can be obtained, for example, by means of sensors such as potentiometers. If a stepper motor is used to drive the throttle valve, it supplies information at the same time on the throttle valve position, so that an additional sensor can be eliminated.
The input signals are used in order to produce an output signal in the control unit to control the position of the control valve. As already explained, due to the mechanical coupling of turbine and compressor it is possible to contain the control valve both in the bypass duct and in the secondary air duct. Increasing throttling of the control valve leads to a reduction of the secondary air stream as well as the secondary air stream.
By means of the information on the position of the control valve as well as the throttle valve it is possible to reach a conclusion as to the combustion air delivered to the internal combustion engine through the air intake tract.
The methods and apparatus heretofore described for the adjustment of the intake air and secondary air permit in various ways to estimate the processes really taking place in the internal combustion engine. With the estimate a control or regulation of the necessary secondary air as well as intake are described sufficiently accurately so that the expenditure for the adjustment of the air streams can be kept within limits. At the same time allowance is made for the main influence factor of the system. This influence factor is the throttle valve in the intake tract. As long as the latter is closed the entire air intake has to be carried through the bypass duct, which results in an excessively high turbine power. As soon as this air flow is no longer sufficient, the throttle valve in the air intake tract is opened, which results in an abrupt decrease of the turbine power. Under these conditions there is no longer any supply of secondary air to the exhaust system. The adjustability of the cross section of the bypass duct or secondary air duct leads, however, to an additional adjustability of the secondary air stream. Thus, if the throttle valve opens in the air intake tract, a reduction of the resistance to flow of the secondary air system can begin. Thus the loss of power at the turbine can be avoided or at least reduced since the mass flow in the bypass duct can be kept approximately constant due to the lowering of resistance to flow. The reduction of the resistance to flow is achieved, for example, through the previously described throttle devices in the secondary air duct or bypass duct.
If the adjustment mechanisms described should no longer suffice to assure the delivery of sufficient secondary air into the exhaust system, the feeding of fuel into the air intake tract can additionally be reduced. Thus the mixture becomes leaner, so that a thorough combustion can take place in the internal combustion engine. Therefore even in this case the exhaust levels of the internal combustion engine can remain compliant.
The secondary air system does not have to be controlled by a separate control unit. The functions can also be integrated into the motor control, thereby improving the economy of the proposed solution. In particular, various factors measured in the internal combustion engine are available for controlling the motor, and they do not have to be detected additionally. Other readings can additionally serve to refine the method for adjusting the secondary air system that has been described.
The fault-free operation of the secondary air system depends to a great extent also on the design of the individual components. The secondary air charger is to be designed such that, when the motor is idling, the necessary secondary air mass flow can be delivered by the compressor. Since the air requirement of the motor in this state of operation is slight, and this low amount of air must be maintained in order to reach the idling speed, the throttling of the turbine depends on these marginal conditions. If the compressor mass flow is too low in the case of turbines throttled in this way, the basic design of the secondary air charger, especially of the turbine must be revised. If the turbine is designed smaller, the throttling of the turbine can be reduced and the turbine pressure ratio increases. For the same turbine mass flow then more turbine power is produced and a greater compressor mass flow is required. The current level of the secondary air mass flow is detected by a sensor in the secondary air duct or in the exhaust system. In this case flow sensors, for example, can be used, and these must often be provided anyway in the exhaust system. For example, a comparison of the air throughput ahead of and behind the introduction of the secondary air would permit knowing the amount of secondary air delivered. Alternatively, a differential pressure at a known throttle point can also be evaluated. The required value of the secondary air mass flow can be determined, for example, by the motor control. In addition, the mass flow of engine air and fuel can be evaluated. The control unit, by applying the described method, adapts the secondary air mass flow to the desired level.
The method described is thus suitable in a special manner for regulating the mass flow of the secondary air flow. According to a special embodiment of the invention, the secondary air mass flow thus produced can be used for the internal combustion engine not just in the cold start phase. For this purpose the secondary air is fed to the exhaust system ahead of the catalyst. This can be done centrally, directly ahead of the catalyst or also in the exhaust passages of the cylinders. In a case where the secondary air is introduced near the cylinder outlet ports a better distribution of the secondary air in the exhaust gas is possible since the latter still has to travel a certain distance to the catalyst.
The secondary air, however, can also be introduced behind the catalyst. This possibility creates an additional application, wherein an adjusting means is provided which can select the delivery of the secondary air to the various inlets.
The application involving the delivery of secondary air behind the catalyst is used in the case of lean-fed internal combustion engines which are equipped with so-called storage catalyst systems for nitrogen oxides. These special catalysts are intended to keep the pollutant emission within the prescribed limits. They store up nitrogen oxides at leaner operating points and yield the nitrogen at rich operating points. The terms, lean and rich, refer to the states of operation in which more fuel or less fuel is admixed to the combustion mixture. In lean operation the fuel burns in the cylinders with excess air, while at the rich points of operation an excess of fuel is injected into the cylinders.
It is true that the sulfur originating from the fuel can limit the described action of the storage catalysts. This is caused by sulfur embedded in the catalyst, which can greatly reduce its running time.
Within certain limits the embedded sulfur, however, can be reversed. For that purpose the motor must be operated rich for a comparatively long time in minutes and at high exhaust gas temperatures. Thus a purification of embedded sulfates from the catalyst can be achieved, yet the carbon monoxide and hydrocarbon emission levels of the exhaust gas increase to unacceptable levels. By the introduction of secondary air downstream from the components to be desulfatized they can be burned, depending on the process, in the cold-start phase of the internal combustion engine. In this way a cleansing of the catalyst can be achieved at acceptable emission levels.
The described secondary air system can be used to produce the state of operation of the desulfatizing of the catalyst. In this case advantage can be taken of the circumstance that the conditions of the cold start and desulfatization never occur simultaneously. The cold-start phase amounts to only a few minutes after the motor is started. At the rich working points of the motor that are needed for desulfatizing the catalyst, the throttle valve in the air intake tract can be at least partially closed. Therefore a sufficient amount of circulated air can be made available to the turbine of the secondary air charger to make a sufficient amount of secondary air available by the compressor.
Of course, other secondary air systems which have, for example, an electrically driven compressor, can be used for the desulfatizing of the catalyst. In the design of the secondary air charger, care must be taken to see that it can supply the required amount of secondary air both in the cold-start phase and during the desulfatization. The secondary air charger must thus be designed for operation with the higher requirement of secondary air. The level of the secondary air requirement depends in the individual case on the internal combustion engine.
These and other features of preferred improvements of the invention will appear not only in the claims but also in the description and the drawings; the individual features each by itself or together in the form of sub-combinations in the embodiment of the invention and in other fields of activity, and can be realized in other fields and can represent advantageously a well as independently patentable embodiments, for which protection is here claimed.